doc: alphabetize options in notmuch-config(1)

Originally (I think) these were in the order generated by notmuch
setup. As the number of options grows, and several are not in the
initial setup generated file, the original order becomes less useful
for users. This commit alphabetizes the keys to help users
search. There is only one content change, an added cross-reference
from user.other_email to user.primary_email.